1. Field of the Invention
The present invention relates to an apparatus and the method for carrying out catalytic distillations using a divided wall column, for example the etherification of isoolefins, particularly C5 isoolefins with methanol to produce the corresponding tertiary ether, wherein catalytic distillation is used in a divided wall catalytic distillation reactor to simultaneously separate tertiary amyl methyl ether (TAME) and react substantially all of the methanol to preclude the use of a separate methanol recovery system.
2. Related Information
A divided wall distillation column or divided wall column is a distillation vessel having a vertical partition separating one side from the other for a portion or all of the height of the vessel. The divided wall column may have a common rectification section, a common stripping section or both. Such divided wall columns are variously described in U.S. Pat. Nos. 4,230,533; 4,582,569; 4,826,574; 5,339,648 and 5,755,933. Engineering design methods are assumed to be used to assure proper distribution of upflowing vapor to the alternate sides of a divided-wall column. Such methods to control vapor split may be active or passive. Also, engineering design methods are assumed to assure the proper controlled split of the liquid to both sides of a divided wall device. Such splits are purposely targeted to accomplish specific design objectives as determined from rigorous simulation analysis of the intended operation.
A specialized use of a distillation column, known as catalytic distillation has been used in etherifications, hydrogenations, hydrodesulfurizations, isomerizations, thioetherifications, oligomerizations and others. The catalytic distillation process employs a catalyst system (see U.S. Pat. Nos. 4,215,011 and 4,302,356) which provides for both reaction and distillation concurrently in the same reactor, at least in part within the catalyst system. The method involved is briefly described as one where concurrent reaction and distillation occur in a combination reactor-distillation structure as described in several U.S. patents, namely U.S. Pat. Nos. 4,242,530; 4,250,052; 4,232,177; 4,302,356; 4,307,254; and 4,336,407.
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in several of the previously cited patents and U.S. Pat. Nos. 4,504,687; 4,987,807; and 5,118,873.
As an example, in a catalytic distillation etherification system the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing suitable catalyst, such as an acid cation exchange resin, preferably in the form of catalytic distillation structure, and also preferably, having a distillation zone containing an inert distillation structure, e.g., trays, saddles, and the like. As embodied in the etherification of iC4='s and/or iC5='s the olefin and an excess of methanol may be first fed to a straight pass reactor wherein most of the olefin is reacted to form the corresponding ether, methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME). The feeds may contain both normal and iso olefins. The reaction is highly selective toward the isoolefins. The straight pass reactor is preferably operated at a given pressure such that the reaction mixture is at the boiling point, thereby limiting the temperature rise across the reactor by permitting the exothermic heat of reaction to partially vaporize the mixture. A straight pass reactor and process are described more completely in U.S. Pat. No. 4,950,803 which is hereby incorporated by reference.
The effluent from the first reactor is then fed to a distillation column reactor wherein the unreacted isoolefins are converted to ether, the excess methanol and unreacted hydrocarbons are withdrawn as an overhead product while the ether is withdrawn as bottoms product stream.
As noted above, in the etherification of olefins with an alcohol there is preferably an excess of the alcohol available. This excess alcohol is typically recovered from the overhead stream in downstream units.
In the case of the C5's system the overhead product will contain the azeotropic level of MeOH which is about 12 wt %. If the net flow of methanol into the column (allowing for that reacting in the column) is less than the azeotrope concentration in the distillate, the methanol concentration in the reaction distillation zone will be relatively quite low, about 1%. If the net methanol flow into the column is higher than the azeotrope, the methanol concentration will increase (60% has been measured) until methanol leaves with the TAME bottoms product. Neither case is desirable because at low concentration the conversion of isoamylene to TAME is low whereas at high concentrations the TAME purity is affected by the presence of the excess methanol.
The methanol feed is thus best controlled to produce the highest methanol concentration within the catalyst bed while preventing methanol leaving with the bottoms product. This results in close to the azeotropic concentration in the distillate product and in the reaction distillation zone. The methanol must be separated from the hydrocarbons so that the hydrocarbons can be used for gasoline blending and to conserve methanol. The separation is usually achieved by washing the hydrocarbon/methanol mixture with water. The methanol is selectively absorbed in the water phase which is subsequently fractionated to separate the methanol.
The recovery of the methanol requires considerable amounts of water energy and significant number of theoretical stages which substantially increases the operating and capital cost of the process. It is an advantage of the present invention that in an etherification embodiment wherein an alcohol azeotrope is formed an alcohol recovery section is not required. It is a further advantage of the present invention that the alcohol/hydrocarbon azeotrope is maintained throughout essentially all of the reaction distillation zone, maximizing conversion of the reactive olefins.